Concentration of solutions: Many chemical reactions take place in solutions.

The idea

A solution forms when a solute dissolves in a solvent, and many reactions occur between dissolved reactants. The concentration of a solution tells you how much solute is present in a given volume of solution. It can be expressed as mass per unit volume, commonly g dm⁻³, or as mol dm⁻³. For example, dissolving 20 g of sodium hydroxide in water and making the volume up to 1 dm³ gives a concentration of 20 g dm⁻³. Reactions in solution are useful because dissolved particles mix freely and can react quickly. Understanding concentration allows you to compare solutions, predict reaction behaviour and calculate amounts of solute in a given volume.

What earns the marks

• Define a solution as a mixture formed when a solute dissolves in a solvent.
• State that concentration describes the amount of solute in a given volume of solution.
• Give common units of concentration such as g dm⁻³ or mol dm⁻³.
• Explain that reactions in solution involve dissolved particles that can mix and collide.
• Use the relationship concentration = mass of solute ÷ volume of solution when carrying out simple calculations.

Common mistakes and how to put them right

• Confusing solute and solvent.

  Put it right: The solute is dissolved and the solvent is the liquid that does the dissolving.

• Thinking concentration depends only on the mass of solute.

  Put it right: The correct view is that volume of solution also matters.

• Using the volume of solvent instead of the volume of solution.

  Put it right: The corrected approach uses the total solution volume.

Exam tips

• Read the unit of concentration carefully and convert volumes to dm³ when needed.
• Write the concentration equation before substituting values to reduce errors.
• Link solution reactions to particle collision ideas when asked to explain rate or reactivity.